1. Technical Field
The present application relates to a nitride-based semiconductor light-emitting element and particularly to a semiconductor light-emitting element having a principal surface which is the m-plane.
2. Description of the Related Art
A nitride semiconductor including nitrogen (N) as a Group V element is a prime candidate for a material to make a short-wave light-emitting element because of its wide bandgap. Among other things, gallium nitride-based compound semiconductors (GaN-based semiconductors) have been researched and developed particularly extensively. As a result, blue light-emitting diodes (LEDs), green LEDs, and semiconductor laser diodes made of GaN-based semiconductors have already been used in actual products.
A GaN-based semiconductor has a wurtzite crystal structure. FIG. 1 schematically illustrates a single cell of GaN. In an AlxGayInzN (x+y+z=1, x≧0, y≧0, z≧0) semiconductor crystal, some of the Ga atoms shown in FIG. 1 may be replaced with Al and/or In atoms.
FIG. 2 shows four vectors a1, a2, a3 and c, which are generally used to represent planes of a wurtzite crystal structure with four indices (i.e., hexagonal indices). The primitive vector c runs in the [0001] direction, which is called a “c-axis”. A plane that intersects with the c-axis at right angles is called either a “c-plane” or a “(0001) plane”. It should be noted that the “c-axis” and the “c-plane” are sometimes referred to as “C-axis” and “C-plane”.
In fabricating a semiconductor device using GaN-based semiconductors, a substrate having a principal surface which is a c-plane, i.e., a (0001) plane, is commonly used as a substrate on which GaN semiconductor crystals will be grown. In a c-plane, however, there is a slight shift in the c-axis direction between a Ga atom layer and a nitrogen atom layer, thus producing electrical polarization there. That is why the c-plane is also called a “polar plane”. As a result of the electrical polarization, a piezoelectric field is generated along the c-axis direction in the InGaN quantum well direction in the active layer. Once such a piezoelectric field has been generated in the active layer, some positional deviation occurs in the distributions of electrons and holes in the active layer due to the quantum confinement Stark effect of carriers. Consequently, the internal quantum efficiency decreases. Thus, in the case of a semiconductor laser diode, the threshold current increases. In the case of an LED, the power consumption increases, and the luminous efficacy decreases. Meanwhile, as the density of injected carriers increases, the piezoelectric field is screened, thus varying the emission wavelength, too.
Thus, to overcome these problems, it has been proposed that a substrate having the principal surface which is a non-polar plane such as a (10-10) plane that is perpendicular to the [10-10] direction and that is called an “m-plane” be used. As used herein, “−” attached on the left-hand side of a Miller-Bravais index in the parentheses means a “bar” (a negative direction index). As shown in FIG. 2, the m-plane is parallel to the c-axis (primitive vector c) and intersects with the c-plane at right angles. On the m-plane, Ga atoms and nitrogen atoms are on the same atomic-plane. For that reason, no electrical polarization will be produced perpendicularly to the m-plane. That is why if a semiconductor multilayer structure is formed perpendicularly to the m-plane, no piezoelectric field will be generated in the active layer, thus overcoming the problems described above.
The “m-plane” is a generic term that collectively refers to a family of planes including (10-10), (−1010), (1-100), (−1100), (01-10) and (0-110) planes. As used herein, the “X-plane growth” means epitaxial growth that is produced perpendicularly to the X plane (where X=c, m, etc.) of a hexagonal wurtzite structure. As for the X-plane growth, the X plane will be sometimes referred to herein as a “growing plane”. A layer of semiconductor crystals that have been formed as a result of the X-plane growth will be sometimes referred to herein as an “X-plane semiconductor layer”.
Thus, for example, an LED which is manufactured using such a substrate that has a non-polar plane can have improved emission efficiency as compared with a conventional device which is manufactured on a c-plane.
Further, as disclosed in APPLIED PHYSICS LETTERS 92 (2008) 091105, for example, a LED which includes an active layer formed on the m-plane has unique emission characteristics which are attributed to the structure of its valence band. The band structure of the m-plane InGaN of a light-emitting layer has a strain which is attributed to a lattice mismatch. Due to this strain, the valence band is split. One of the split valence bands which has the highest energy has a track which is similar to px, and therefore, light which is polarized in the a-axis direction is radiated. This polarized light has a possibility that the energy efficiency is greatly improved when employed in a backlight of a liquid crystal device.
Also, to improve the light extraction efficiency of a conventional c-plane LED, for example, a manufacturing method has been proposed which includes performing wet etching on the rear surface of a growth substrate so as to form recesses and protrusions (Japanese Laid-Open Patent Publication No. 2009-218569).
As for the size of a transparent structure portion, a configuration has been proposed in which the ratio between the horizontal dimension and the dimension along the thickness direction (aspect ratio) is not less than 5 and which has a light-scattering function on the surface of a light-emitting element chip or in the inside of the transparent structure portion (Japanese Laid-Open Patent Publication No. 2007-273506).
Further, a gallium nitride-based compound semiconductor light-emitting element including stacked layers of gallium nitride-based compound semiconductors on a substrate, characterized in that a light extraction surface is formed by a transparent film, and a surface of the transparent film has recesses and elevations that have been formed by flat surfaces inclined with respect to the substrate surface, has been proposed (Japanese Laid-Open Patent Publication No. 2006-294907).
Further, a light-emitting element including a transparent inorganic element and a phosphor, characterized in that a first layer, a second layer, and a third layer which has a smaller refractive index than the second layer are stacked, and that the first layer is an aggregate of dots which have the shape of a shell-like lens and is placed on the light-emitting element, has been proposed (Japanese Laid-Open Patent Publication No. 2004-363343).